This study investigates the coupled effects of swirl ratio (SR) and start of injection on diesel–natural gas reactivity-controlled compression ignition (RCCI), taking into account indicated
combustion characteristics and a detailed analysis of emissions. Experiments were conducted on a single-cylinder research engine at a representative operating point (1500 rpm, 0.8 MPa
IMEP, 80% natural gas energy fraction), using four distinct intake port configurations to generate swirl. Diesel injection timing was swept between 34–47 crank angle degrees (CAD)
before top dead centre, constrained by misfire limits. In-cylinder pressure-based thermodynamic analysis and CFD-supported flow-field diagnostics show that swirl has a limited effect on cylinder-bowl-initiated ignition but significantly influences combustion development as it progresses into the squish region. A high SR advanced combustion phasing by 5.4 CAD and increased the peak heat release rate by 20%, but reduced combustion efficiency by up to 4 percentage points compared with low-swirl configurations. Excessive swirl-induced mixture over homogenisation and wall heat losses, increasing unburnt hydrocarbon and CO emissions by a factor of two to four. NOX emissions correlated strongly with combustion timing and temperature, while remaining largely insensitive to SR. There was no direct trade-off between NOX and particulate emissions, with particulate mass remaining ultra-low. However, more premixed conditions (early injection and large swirl) favoured the production of fine particles, resulting in particulate number emissions exceeding the Stage V regulatory limit by a factor of two. Optimal performance was achieved with the all-ports-open configuration, which combined the lowest tumble and swirl. This setup yielded the highest thermal efficiency, nearly 44%, and the lowest combined emissions. The results establish that a swirl–tumble balance, not swirl intensity alone, is key for robust RCCI operation, with tumble particularly relevant for broadening theignitability window.